1. Field of the Invention
The present invention relates to the treatment of wastewater, such as, for example, municipal, industrial or concentrated animal feeding operation (CAFO) wastewaters. This invention is especially useful in facilitating direct removal of contaminants from wastewaters and enhancement of biological processes for treating wastewater.
2. Prior Art
The treatment of contaminated wastewater from municipal, industrial or CAFO sources involves a sequence of processing steps for maximizing water purification at minimum costs. Industrial effluents, particularly wastewater from oil refineries and chemical factories, include a broad spectrum of contaminants, and, consequently, such wastewater is usually more difficult to decontaminate than wastewater from municipal sewage systems. Four main sequential process treatments are used to decontaminate such industrial effluents although similar treatment is given municipal effluents, or combined municipal/industrial effluents. These are primary, intermediate, secondary, and tertiary treatments. The primary treatment calls for removal of gross amounts of oil and grease and solids from the wastewater. In municipal and CAFO wastewater treatment, generally little free oil is present but solids removal is still needed. The intermediate treatment is the next process and it is designed to adjust water conditions so that the water entering the secondary treatment zone will not impair the operation of the secondary treatment processes. In other words, intermediate treatment is designed to optimize water conditions so that the secondary treatment process will operate most efficiently. The secondary treatment calls for biologically degrading dissolved organics and ammonia in the water. One of the most common biological treatment processes employed is the activated sludge process discussed below in greater detail. The tertiary treatment calls for removing residual biological solids present in the effluent from the secondary treatment zone and further removing trace contaminants which contribute to impairing water clarity or adversely affecting water taste and odor. This is usually a filtration of the water, preferably through beds of sand, or combinations of sand and coal, followed by treatment with activated carbon.
The activated sludge process is a conventional wastewater treating process which produces a high degree of biological treatment in a reasonably compact format. The application of this process to the treatment of industrial and CAFO wastewater has, however, been relatively slow compared with municipal applications. Industrial applications of this process are nevertheless increasing rapidly. Currently, the activated sludge process is capable of reliably achieving about 85% to 98% reduction in the five-day biochemical oxygen demand (BOD.sub.5). However, the BOD.sub.5 contaminants present in industrial wastewater are typically small compared with the total oxygen demanding contaminants present in such wastewater as measured by the chemical oxygen demand (COD) test. For example, the BOD.sub.5 contaminants present in the effluent from an activated sludge process typically ranges from 2 to 20 parts per million parts of water. It is not uncommon to also find present in such effluent 10 to 20 times this amount of COD.
The activated sludge process generally has at least two, but preferably four stages of treatment. In the first stage, contaminated water is contacted with the activated sludge. The sludge includes microorganisms which feed on the contaminants in the water and metabolize those contaminants to form cellular structure and intermediate products. This decontaminated water flows into a second clarifier stage where suspended sludge particles are separated from the decontaminated water. A portion of the sludge is recycled to the first stage and the remainder can be forwarded to the third and fourth stages as is taught in Grutsch et al., U.S. Pat. Nos. 4,073,722 and 4,292,176. This sludge forwarded to the third and fourth stages includes water. In the third stage the sludge is thickened to remove excess water and in the fourth stage the thickened sludge is permitted to digest, that is, the microorganisms feed upon their own cellular structure and are stabilized. The digestion step stabilizes the microorganisms. U.S. Pat. No. 4,073,722 teaches dewatering, thickening, and digestion of activated sludge and powdered activated carbon mixtures.
Activated carbon is often used in tertiary treatment as a final cleanup for water discharged from the second stage clarifier. Some have taught that activated carbon, fine carbon (e.g., powdered anthracite) or fine particle clays such as bentonite and Fuller's earth can be used to treat wastewater in a biological treatment process. U.S. Pat. No. 3,904,518 teaches that between about 50 and 1500 parts of activated carbon or between about 250 and 2500 parts of adsorptive bentonite or Fuller's earth per million parts of feed wastewater can be beneficial in water purification. The carbon or Fuller's earth has a surface area of at least 100 square meters per gram and the activated carbon will typically have a surface area of between 600-1400 square meters per gram.
While a variety of powdered inorganic adsorbent materials have been used in biological treatment systems, powdered activated carbon remains the material most commonly used. Several mechanisms have been suggested as to how these materials enhance the treatment of wastewater: improved buffering; increased biological surface area for key organisms such as nitrifying bacteria that favor attached growth; decreased system sensitivity to toxic substances; improved phase separation, and adsorption. Adsorption is most important when the system is operated at low solids retention times (SRT)—namely, before particles are colonized by attached growth bacteria and other microorganisms. Thus, as particle surface area becomes less accessible, the role of adsorption decreases and the other mechanisms dominate.
The relatively high cost of activated carbon has served as a strong deterrent to common use of the material in activated sludge treatment of municipal, industrial and CAFO wastewaters. One approach to reducing cost of activated carbon has been recovery, regeneration and recycling of the activated carbon. This is best illustrated in the appropriately labeled Powdered Activated Carbon Treatment (PACT.TM.) system disclosed in U.S. Pat. Nos. 3,904,518 and 4,069,148, by Hutton et al. The PACT.TM. treatment system operates as a continuous flow process with an aeration basin followed by a discrete clarifier to separate biologically active solids and carbon from the treated wastewater. The recovered, regenerated powdered activated carbon is then returned to the aeration basin along with a portion of the recovered sludge.
Rehm and coworkers have further refined the use of activated carbon in the aerobic oxidation of phenolic materials by using microorganisms immobilized on granular carbon as a porous biomass support system. Utilizing the propensity of microorganisms to grow on and remain attached to a surface, Rehm used a granular activated carbon support of high surface area (1300 m.sup.2/g) to which cells were attached within the macropores of the support and on its surface, as a porous biomass support system in a loop reactor for phenol removal. H. M. Ehrhardt and H. J. Rehm, Appl. Microbiol. Biotechnol., 21, 32-6 (1985). The resulting “immobilized” cells exhibited phenol tolerance up to a level in the feed of about 15 g/L, whereas free cells showed a tolerance not more than 1.5 g/L. It was postulated that the activated carbon operated like a “buffer and depot” in protecting the immobilized microorganisms by adsorbing toxic phenol concentrations and setting low quantities of the adsorbed phenol free for gradual biodegradation. This work was somewhat refined using a mixed culture immobilized on activated carbon [A. Morsen and H. J. Rehm, Appl. Microbiol. Biotechnol., 26, 283-8 (1987)] where the investigators noted that a considerable amount of microorganisms had “grown out” into the aqueous medium, i.e., there was substantial sludge formation in their system.
Suidan and coworkers have done considerable research on the analogous anaerobic degradation of phenol using a packed bed of microorganisms attached to granular carbon [Y. T. Wang, M. T. Suidan and B. E. Rittman, Journal Water Pollut. Control Fed., 58 227-33 (1986)]. For example, using granular activated carbon of 16 times 20 mesh as a support medium for microorganisms in an expanded bed configuration, and with feed containing from 358-1432 mg phenol/L, effluent phenol levels of about 0.06 mg/L (60 ppb) were obtained at a hydraulic residence time (HRT) of about 24 hours. Somewhat later, a beri-saddle-packed bed and expanded bed granular activated carbon anaerobic reactor in series were used to show a high conversion of COD to methane, virtually all of which occurred in the expanded bed reactor; P. Fox, M. T. Suidan, and J. T. Pfeffer, ibid., 60, 86-92 (1988). The refractory nature of ortho- and meta-cresols toward degradation also was noted.
The impregnation of flexible polymeric foams with activated carbon is known to increase the ability of fabrics and garments to resist the passage of noxious chemicals and gases see for example, U.S. Pat. Nos. 4,045,609 and 4,046,939. However, these patents do not teach the use of these foams in wastewater treatment, or that these foams are a superior immobilization support for the growth and activity of microorganisms.
Givens and Sack, 42nd Purdue University Industrial Waste Conference Proceedings, pp. 93-102 (1987), performed an extensive evaluation of a carbon impregnated polyurethane foam as a microbial support system for the aerobic removal of pollutants, including phenol. Porous polyurethane foam internally impregnated with activated carbon and having microorganisms attached externally was used in an activated sludge reactor, analogous to the Captor and Linpor processes which differ only in the absence of foam-entrapped carbon. The process was attended by substantial sludge formation and without any beneficial effect of carbon.
The Captor process itself utilizes porous polyurethane foam pads to provide a large external surface for microbial growth in an aeration tank for biological wastewater treatment. The work described above is the Captor process modified by the presence of carbon entrapped within the foam. A two-year pilot plant evaluation of the Captor process itself showed substantial sludge formation with significantly lower microbial density than had been claimed. J. A. Heidman, R. C. Brenner and H. J. Shah, J. of Environmental Engineering, 114, 1077-96 (1988). A point to be noted, as will be revisited below, is that the Captor process is essentially an aerated sludge reactor where the pads are retained in an aeration tank by screens in the effluent line. Excess sludge needs to be continually removed by removing a portion of the pads via a conveyor and passing the pads through pressure rollers to squeeze out the solids.
In U.S. Pat. No. 6,395,522 DeFilippini and coworkers describe a biologically active support system for providing removal of pollutants such as aliphatics, aromatics, heteroaromatics and halogenated derivatives from waste streams. The support contains a particulate adsorbent such as activated carbon bound by a polymer binder to a substrate such as a polymeric foam, and a bound pollutant-degrading microorganism. They claim that the biologically active support can be used in conventional aerobic biological waste treatment systems such as continuous stirred reactors, fixed-bed reactors and fluidized bed reactors.
H. Bettmann and H. J. Rehm, Appl. Microbial. Biotechnol., 22, 389-393 (1985) have employed a fluidized bed bioreactor for the successful continuous aerobic degradation of phenol at a hydraulic residence time of about 15 hours using Pseudomonas putida entrapped in a polyacrylamide-hydrazide gel. The use of microorganisms entrapped within polyurethane foams in aerobic oxidation of phenol in shake flasks also has been reported; A. M. Anselmo et al., Biotechnology B.L., 7, 889-894 (1985).
Known bioremediation processes suffer from a number of inherent disadvantages. For example, a major result of increased use of such processes is an ever increasing quantity of sludge, which presents a serious disposal problem because of increasingly restrictive policies on dumping or spreading untreated sludge on land and at sea. G. Michael Alsop and Richard A. Conroy, “Improved Thermal Sludge Conditioning by Treatment With Acids and Bases”, Journal WPCF, Vol. 54, No. 2 (1982), T. Calcutt and R. Frost, “Sludge Processing—Chances for Tomorrow”, Journal of the Institute of Water Pollution Control, Vol. 86, No. 2 (1987) and “The Municipal Waste Landfill Crisis and A Response of New Technology”, Prepared by United States Building Corporation, P.O. Box 49704, Los Angles, Calif. 90049 (Nov. 22, 1988). The cost of sludge disposal today may be several fold greater than the sum of other operating costs of wastewater treatment.
A slightly different biophysical treatment process is described by McShane et al., in “Biophysical Treatment of Landfill Leachate Containing Organic Compounds”, Proceedings of Industrial Waste Conference, 1986 (Pub. 1987), 41st, 167-77. In this process a biological batch reactor is used with powdered activated carbon and the system is operated in the “fill and draw” mode, also known as the sequenced batch reactor (SBR) mode. A similar scheme for treatment of leachate is disclosed in U.S. Pat. No. 4,623,464 by Ying et al. in which an SBR is operated with both biologically active solids and carbon present to treat a highly toxic PCB and dioxin-containing leachate. Supplementation with powdered activated carbon has been successfully demonstrated to improve treatment of widely differing wastewater streams in all such variations of the activated sludge process. Use of powdered activated carbon in this manner remains, nevertheless, rare—particularly in treatment of municipal wastewaters where cost factors are paramount. Most wastewater treatment plant owners and treatment plant managers and system operators deem the cost of doing so to be excessive.
The literature and prior art does not provide any instruction on substituting renewable powdered natural lignocellulosic materials for the carbon and clays and other inorganic and manufactured adsorbents now deemed to be too costly by potential users. A number of such materials, notably kenaf core, have available surface area to weight ratios that are comparable to activated carbon and greatly exceed those of fine clays and carbon dust. Because they are generally cropped materials, these renewable resources can be delivered at much lower cost than activated carbon. Because they are biodegradable, they can also easily be disposed of along with the other sludge constituents. As such, these natural materials hold potential to succeed in achieving widespread usage where activated carbon, clays and carbon dust have not succeeded.